(1) Field of the Invention
The present invention relates to fire retardant compositions, and more particularly to fire retardant compositions comprising the salt of an acidic monomer grafted onto a polymer substrate.
(2) Description of the Prior Art
Potential flammability is a problem associated with virtually all polymers. This is especially significant because of the widespread use of polymers such as synthetic fabrics and plastics. Because of the versatility of polymers such as plastics, these materials have found a place in nearly every aspect of daily living. Many of the applications of plastics involve a substantial fire hazard and as a result, the formulation of materials that minimize the risk of fire is of great importance.
A number of approaches to flame retardation of a polymeric material have been tried, none of which is totally satisfactory. Such approaches include addition of a substance that will cause the degradation pathway to change so that volatile gases are not produced; addition of a substance that is effective in the gas phase to remove the species responsible for the flame; and addition of a material that will decompose endothermically and thus remove heat from the system.
Methods involving the addition of substances to change the degradation pathway can be cumbersome or of limited application since they require that enough information be known about the effects of different additives on a specific polymer in order to design a suitable additive for that polymer. The information that is developed for one polymer generally cannot be extended to other polymers.
Gas-phase additives such as halogen-containing compounds are well known to extinguish the fire by interacting in the gas phase with the radicals that make up the flame. This approach to flame retardation has the advantage of being applicable to a wide variety of polymers. However, when halogen-based compounds are used, hydrogen halides along with other toxic compounds such as dioxins may be produced which could create a toxic environmental hazard. Thus, this approach to making a polymer flame retardant is far from ideal.
Endothermic materials, such as hydrated alumina or magnesium hydroxide, decompose and remove heat from the polymer and thus slow the combustion process. They are applicable to a variety of polymers, but their effect is limited. Moreover, because they are in the nature of a filler for the polymer they may impart undesirable characteristics to the polymer.
The two most prominent chemical classes of flame retardants are those based on halogen compositions and those based on phosphorus compositions. The mode of action of halogen-based flame retardants is thought to involve inhibition of gas-phase chain reactions. Phos-phorus-based flame retardants are less well understood. However, it is thought that this class of compounds can act both to alter the degradation pathway to produce less volatile gases as well and to effect a removal of flammable species from the gas phase. The effectiveness of a particular phosphorus composition depends upon the nature of the substrate polymer. For example, elemental phosphorus is an effective flame retardant only for oxygen-containing polymers such as polyesters, polyamides, and polyurethanes (see the following publications which are incorporated by reference: Granzow, A., Flame Retardation by Phosphorus Compounds, Accounts of Chemical Research 11(5), 177-183, 1978; Granzow, A. and Cannelongo, J. F., J. Appl. Polym. Sci., 20, 589 et seq., 1976; Granzow, A., Ferrillo, R. G., and Wilson, A., J. Appl. Polym. Sci. , 21, 1657 et seq., 1977).
In other techniques, the polymer itself is flammable, but forms a heat insulative char. This approach of course, requires a suitable polymer and results in the destruction of the polymer.
Thus, such conventional approaches to making a polymer flame retardant have been unsuccessful in developing an ideal composition. Hence, there remains a continuing need for an effective approach to fire retardation that would be generally applicable to most polymers without the production of halogenated compounds.
McNeill and coworkers reported that they have examined the degradation of homopolymers of methacrylic acid and a variety of its salts and have shown that many of these salts will produce a large amount of non-volatile char. See I. C. McNeill and M. Zulfiqar, Polym. Degrad. Stab., 1, 89 et seq., 1979 incorporated herein by reference. The char that is formed from, for example, sodium methacrylate, reportedly consists of sodium carbonate with some elemental carbon. The article describing this work does not disclose the application of compositions to the surface of a polymer, nor does it mention the applicability of the compositions as fire retardants.
In other settings, however, techniques have been employed for grafting certain monomers to polymer substrates for purposes other than fire protection. Graft copolymers are comprised of a long "backbone" sequence of one monomer with one or more branches or "grafts" of a sequence of one or more units of a second monomer. Techniques for synthesizing graft copolymers are well known in the art and usually involve radical graft polymerization although approaches to making graft copolymers including anionic graft polymerization are known and used. For example, see Odian, G. in "Principles of Polymerization", 3rd Ed., Wiley-Interscience, pp. 719-745, 1991, incorporated herein by reference.
Grafting is performed typically to modify the surface properties of the substrate polymer. Thus, a polymer having a hydrophobic surface may be converted to one having a hydrophilic surface by grafting on a hydrophilic copolymer. Such grafted polymers find use in the fabric industry in altering the wetability nature of the fabric and/or in facilitating the dying of the fabric For example see Hebeish, A., Shalaby, S. E., and Bayzeed, A.M., kolorisztikai Ertesito, 2, 74 et seq., 1979; Tolado, G. C., Munari, S., and Calgari, S., Chim. Ind. (Milan), 52, 759 et seq., 1970; Okada, T., and Sakadura, I., Nippon Genshiryoku Kenkyusho Nempo, JAERI, 5026, 63 et seq., 1970; all of which are incorporated herein by reference. None of these references mentions or appears to be applicable in a fire-retardant application.
The grafting of monomers onto various substrates has been reported by others. Vinyl monomers reportedly have been grafted onto cellulose or its derivatives. For example, see Ang, C. H., Garnett, J. L., Jankiewicz, S. V., and Sanger, D., J. Am. Chem. Soc., 10, 141-154, 1982; Akira, R., Yasusato, and Atsushi, K., Sen'i Gakkaishi, 40, T445-T451, 1984; Bottom, R. A., Green, P, and Guthrie, J. T., Polym Photochem, 6, 11 et seq., 1985; all of which are incorporated herein by reference. The grafting of methacrylamide as well as methacrylic acid monomers onto styrene-butadiene-styrene plastic has been reported in Geuskens, G. and Kanda, M. N., Surface Modification of Polymers-I. Grafting Initiated by photo-generated hydroperoxides; Eur. Polym. J.,27, 877-879, 1991; and Geuskens, G. et al., Surface Modification of Polymers-II. Photo-Oxidation of SBS Containing Anthracene and Grafting Initiated by Photo-Generated Hydroperoxides, Eur. Polym. J., 29, 351-355, 1993; and onto acrylonitrile-butadiene-styrene in Abdel-Razik, E. A., Photochem. Photobiol. A Chem., 69, 121-124, 1992; all incorporated herein by reference. None of these references, however, reports the formation of polymer resins with a graft of salt of an acidic monomer onto the polymer or a concern for the char produced by the grafted layer on burning, desire for increasing char thickness, or use of the char layer as a fire retardant.